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Semihermetic compressors.

This is a loaded question, and am sure answer varies, but here goes....

How long should a new Copeland Semihermetic compressor last in a
refigeration system.

I have two Heatcraft refigeration systems in a warehouse, one a 20 ton
system and the other a 30 ton system. This warehouse not insulated
that well, so systems run majority of time. There is actually a third
system that is a 12 ton system, but not talking about this one now.

The 20 ton system keeps room at 58 degrees, and 30 ton keeps room
at 36 degrees. Both systems use R22. The 20 ton system compressor
died last fall when system was 4 years 10 months old, had to replace
compressor. When I drained the oil there were a lot of brass shavings
in the oil, so bearings must have given out.

Today the 30 ton compressor died, locked up and blew fuses. Had my
helper go check the windings one shorted, one high ohms. These
compressors are 3 ph 460 volt.

Both of these units were working flawlessly, never had any problems,
check them once a year, clean evap coils, check pressures, never any
problems, the 30 ton runs 95% of the time, the 20 ton about 70% of
the time.

Is 5 years about how long units that run most of the time should last,
or is this just a unusual event. I have had other semihermetics fail,
but they had a reason to fail. These two times everything normal.

The oil minders were working fine, and both units had phase protectors
that worked. No reason for them to both fail, other that a critical part
gave out. We know the 20 ton failed because of bearing failure, right
now do not know why 30 ton failed yet, it was 109 degrees outside
when unit died. Oil up to half way mark on sight glass, an oil is clear.

I find semi's to be very reliable.
Looking at the size it would seem these compressors, are lubricated by a positive pressure oil pump.
So my gut feeling is that you have some shtye in the oil channels ways feeding the bearings. I presume you have a low oil pressure diff switch fitted, that works!
Being R22 high discharge temps can be a problem (not normally at your stated room temps), but if you had high compressor suction superheat, then high discharge temps could cause so breakdown of the oil.
Any discoloration of the comp heads?
Does the system run for very long periods unloaded?